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arXiv:1901.09882 · Physical Review Letters

Laser Cooling of Radium Ions

Mingyu Fan, C. A. Holliman, A. L. Wang, Andrew M. Jayich

Published Jun 7, 2019• 28 citations
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Abstract

The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation, and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity (∼10 μCi) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the 7p^{2}P_{1/2}^{o} state's branching fractions to the ground state, 7s^{2}S_{1/2}, and a metastable excited state, 6d^{2}D_{3/2}, to be p=0.9104(7) and 0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s^{2}S_{1/2}→7p^{2}P_{1/2}^{o} transition frequency, 640.096 63(6) THz.

DOI
10.1103/physrevlett.122.223001

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The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation, and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity (∼10 μCi) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the 7p^{2}P_{1/2}^{o} state's branching fractions to the ground state, 7s^{2}S_{1/2}, and a metastable excited state, 6d^{2}D_{3/2}, to be p=0.9104(7) and 0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s^{2}S_{1/2}→7p^{2}P_{1/2}^{o} transition frequency, 640.096 63(6) THz.
Measurement of 7p 2P1/2o7p\ ^2P_{1/2}^o7p 2P1/2o​ branching fractions with laser-cooled 226Ra+^{226}\mathrm{Ra}^+226Ra+
The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a table-top experiment with a low-activity (∼10μCi\sim 10 \mu\mathrm{Ci}∼10μCi) source where we laser-cool radium ions to form radium Wigner crystals. With a laser-cooled radium ion we measured the 7p2P1/2o7p ^2P_{1/2}^o7p2P1/2o​ state's branching fractions to the ground state, 7s2S1/27s ^2S_{1/2}7s2S1/2​, and a metastable excited state, 6d2D3/26d ^2D_{3/2}6d2D3/2​, to be p=0.9104(7)p=0.9104(7)p=0.9104(7) and 0.0896(7)0.0896(7)0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s2S1/2→7p2P1/2o7s ^2S_{1/2} \rightarrow 7p ^2P_{1/2}^o7s2S1/2​→7p2P1/2o​ transition frequency, 640.09663(6) THz.
We report laser cooling and trapping of 224Ra+{}^{224}{\mathrm{Ra}}^{+}224Ra+ ions. This was realized via two-step photoionization loading of radium into an ion trap. A robust source for 224Ra^{224}\mathrm{Ra}224Ra atoms, which have a 3.6-day half-life, was realized with an effusive oven containing 228Th^{228}\mathrm{Th}228Th, which has a 1.9-yr half-life, which continuously generates 224Ra^{224}\mathrm{Ra}224Ra via its $\ensuremath{\alpha}$-decay. We characterized the efficacy of this source and found that after depleting built-up radium the thorium decay provides a continuous source of radium atoms suitable for ion trapping. The vacuum system has been sealed for more than 6 months and continues to trap ions on demand. We also report a measurement of the $^{224}\mathrm{Ra} 7{s}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{S}}_{0}\ensuremath{\rightarrow}7s7p{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{P}}_{1}$ transition frequency: $621\phantom{\rule{0.16em}{0ex}}043\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ MHz, which is helpful for efficient photoionization. With this measurement and previous isotope shift measurements we find that the frequency of the same transition in 226Ra^{226}\mathrm{Ra}226Ra is $621\phantom{\rule{0.16em}{0ex}}037\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ MHz, which disagrees with the most precise measurement, $621\phantom{\rule{0.16em}{0ex}}038\phantom{\rule{0.16em}{0ex}}489\ifmmode\pm\else\textpm\fi{}15$ MHz, which is used for the recommended value in the National Institute of Standards and Technology Atomic Spectra Database.
Radioactive molecules have remained severely understudied relative to their stable counterparts, owing to difficulties associated with their radiological hazards and limited material availabilities. Despite this, their relevancy to many fields including nuclear science, chemistry, and astronomy make them attractive prospects for precision investigations. In particular, laser-coolable radioactive molecules containing octupole-deformed nuclei such as radium monohydroxide (RaOH) and radium monofluoride (RaF) promise dramatic sensitivity increases for experiments aiming to measure new physics originating from fundamental symmetry violations. Before they may be used for any of these purposes, however, new tools are needed that allow for their safe and precise study while making them accessible in a university (or similar) setting. In this thesis, we construct a tabletop cryogenic apparatus capable of studying radioactive molecules which we produce in-situ from trace quantities of starting material. We use this apparatus to perform the first spectroscopic observation of a radium-containing polyatomic molecule RaOH and its deuterated isotopologue RaOD, as well as high-resolution spectroscopy of the photon cycling transitions in RaF. To prepare for laser cooling, we develop a protocol to measure the vibrational branching ratios of these exotic species and use it to establish RaF to be among the most laser-coolable molecules known as of yet. We demonstrate the versatility of our approach by producing and detecting over twenty different atoms and molecules, ranging from transuranic radioisotopes to complex asymmetric top molecules. The species-agnostic methodology employed throughout this work allows its application to a wide class of previously ignored radioactive atoms and molecules, and makes them accessible to the many advanced experimental techniques already leveraged in stable systems.
This thesis is written as a part of the TRImP programme. Rare and short-lived radioactive isotopes offer new possibilities for investigating fundamental interactions and symmetries. Radium, a radioactive alkaline earth element has been identified as a new candidate for searches for a permanent electric dipole moment in neutral atoms. The TRImP facility at KVI, Groningen, The Netherlands is being setup to produce and utilize these radioactive isotopes to study fundamental interactions and symmetries. We use atom traps for these studies. We presented the various stages for the production, separation and slowing of the short-lived radioactive isotopes of our interest at the TRImP facility. We need to expand laser cooling and trapping methods to heavy alkaline earth elements barium and radium. Stable barium is well suited as a precursor for radioactive radium, because of its similar level scheme and chemical properties. Because of the radioactivity of radium we have chosen to prepare the cooling and trapping with the homologue barium. Stable isotopes of barium were used in the present study. For spectroscopy, metastable singlet and triplet D-states were populated by optical methods rather than using the conventional method of atomic discharges. The singlet D-state is populated by driving the 1S0-1P1 transition. To populate the triplet D-states efficiently the branching ratio of the 3P1 level was exploited by optical pumping with the intercombination line using the diode lasers at 791.3 nm. Long term frequency stability of these lasers were achieved by referencing the frequency to a nearby transition in molecular iodine. We performed first direct optical spectroscopy of the 1D2-1P1(1500.4 nm), 3D1-1P1(1107.8 nm) and 3D2-1p1 (1130.6 nm) transitions respectively. The hyperfine structure of the 3D1 and 3D2 levels for the Ba-135,137 isotopes were optically resolved. In addition, isotope shifts of the 3D1-1P1 transition were measured. We report the experimental observation of coherent Raman resonances in barium. The relevant Rabi frequencies were extracted for the lambda system and estimated the transition rates, relevant for the laser cooling experiment. We have achieved the first slowing of a barium atomic beam. These results can be applied for atoms, in which a closed cooling cycle can only be found with several laser fields at different wavelengths.
We have developed an apparatus integrating resonance-ionization, ion-trap, and laser-cooling techniques for an ultratrace radioactive isotope 90Sr^{90}\mathrm{Sr}90Sr analysis. Trapped 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ isotope ions were laser cooled, and their $4d\phantom{\rule{0.16em}{0ex}}^{2}D_{3/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ transition isotope shift was experimentally measured to be $\ensuremath{-}281(17)$ MHz by comparing individual spectra of 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ and 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ ions. Crystallization of 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ was carried out using the resonance frequency value confirmed in our experiment, and then 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ individual ions were successfully observed.
Abstract Recently, a few diatomic and polyatomic molecules have been identified as prospective systems for Doppler/Sisyphus cooling. Doppler/Sisyphus cooling allows us to decrease the kinetic energy of molecules down to μ K temperatures with high efficiency and allows their capture in molecular traps, including magneto-optical traps. Trapped molecules can be used for creating molecular fountains and/or performing controlled chemical reactions, high-precision spectra measurements, and a wealth of other applications. Polyatomic molecules with heavy nuclei are of considerable interest for the ‘new physics’ search outside of the Standard Model and other applications including cold chemistry and photochemistry, quantum informatics, etc. Here, we focus on the radium monohydroxide molecule (RaOH), which is on the one hand amenable to laser cooling and on the other hand opens excellent possibilities for  -odd and  ,  -odd effects research. At present, RaOH is the heaviest polyatomic molecule proposed for direct cooling with lasers.